High capacity anodes

ABSTRACT

A novel anode for a lithium battery cell is provided. The anode contains silicon nanoparticles embedded in a solid polymer electrolyte. The electrolyte can also act as a binder for the silicon nanoparticles. A plurality of voids is dispersed throughout the solid polymer electrolyte. The anode may also contain electronically conductive carbon particles. Upon charging of the cell, the silicon nanoparticles expand as take up lithium ions. The solid polymer electrolyte can deform reversibly in response to the expansion of the nanoparticles and transfer the volume expansion to the voids.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application61/085,790, filed Aug. 1, 2008 and to International Patent ApplicationNumber PCT/US09/52511, filed Jul. 31, 2009, both of which areincorporated by reference herein.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates generally to electrodes for batteries, and, morespecifically, to high capacity nanostructured anodes for lithiumbatteries.

There has been much interest in lithium batteries as they have thehighest specific energy (up to 180 Wh/kg) and energy density (up to 550Wh/L) among chemical and electrochemical energy storage systems.Currently, lithium ion batteries are used for portable electronics, suchas laptops and cell phones. There may also be use for lithium ionbatteries in transportation systems, such as bicycles or automobiles.

Graphitic carbon has been the negative electrode material of choice forconventional Li-ion batteries. Graphitic carbon is distinguished by itsstable cycle properties and its very high safety. In a conventionalLi-ion cell, the anode is made from graphitic carbon, the cathode is ametal oxide, and the electrolyte is a lithium salt in an organicsolvent. The typical cathode half reaction is:

The anode half reaction is:

The graphitic intercalation anode allows Li ions to intercalate andde-intercalate reversibly at relatively low potentials (0-0.3 V) withrespect to Li/Li⁺. At such low potentials, the organic electrolyte isreductively unstable, but in the first few cycles it undergoes a surfacereaction with the graphite and with impurities and/or electrolyte toform a so-called solid electrolyte interface (SEI) layer on thegraphitic surface. The SEI layer is ionically conductive and allowstransfer of ions between the electrolyte and the underlying graphite.The SEI also protects against further reduction of the electrolyte.

Graphitic carbon in a negative electrode undergoes a relatively small(compared to pure Li metal) volume change (<40%) during theinsertion/extraction of lithium, i.e., the electrode remains stable overmultiple cycles. However, graphitic carbon's very low potential (about100-200 mV) relative to Li/Li⁺ is disadvantageous. A further drawback tographitic carbon is its relatively low charge capacity (about 372 mAh/gor 818 Ah/L of graphite), which is only about one tenth the theoreticalelectrochemical charge capacity achievable with lithium metal (3862mAh/g and 2047 Ah/L of lithium).

Lithium can form well-defined intermetallic/intercalation phases(Li_(x)M) with numerous metals M (M=A1, Si, Ge, Sn, Pb, Sb, Mg, etc.),their alloys (Si—Sn, Cu—Sn, Sb—Sn, etc.) and with metal oxides (SnO₂,etc.) at fairly low potentials with respect to Li/Li⁺ at roomtemperature. In general the reversible “lithiation” reaction can bewritten as:

The effort to make lithium-ion batteries with high specific energy,these materials have been investigated as promising negative electrode(anode) materials. In particular, silicon-based alloys with hightheoretical specific capacity (e.g., nearly 4200 mAh g⁻¹ for Li₂₂Si₅)have been studied extensively as a replacement for graphite. Yet batterycells that use M-lithium alloys as active anode material have not beenable to maintain their high capacity on prolonged cycling.

Upon lithium insertion and extraction, many of these metals and theiralloys undergo significant changes in volume, ranging from more than200% to even as much as 400% in some cases. Thus repeatedcharge/discharge cycles often result in cracking of active materialand/or electrolyte binder and the resulting loss of electricalconduction paths.

In U.S. Patent Application Publication 2007/0202403, published Aug. 30,2007, Oh et al. describe using a nanocomposite binder made of carbonnanotubes in a photo- or thermo-polymerizable material with silicon- ortin-based anode active material particles as an anode in a lithiumbattery. They state that such a binder enables stable maintenance ofadhesion between the anode active material particles as they undergosignificant volume changes during charge/discharge battery cycles,thereby preventing volume changes during cycling. Yet if it werepossible to prevent expansion of the anode active material particles, itwould not be possible for the lithium ions to insert themselves into theparticles; it would not be possible to charge the battery. A liquid orsolid electrolyte is used in the anode. If the anode active materialparticles do expand, no teaching is given as to how the extra volumewould be accommodated.

In U.S. Patent Application Publication 2008/0044732, published Feb. 21,2008, Salot et al. describe an anode for a lithium battery that hassilicon nanowires extending from a current-collecting substrate. A solidelectrolyte rests on the free ends of the nanowires creating closedcavities between the nanowires. Care is taken to be sure that thecavities are large enough to accommodate expansion of the nanowiresduring lithium insertion. Although this may solve some of the problemsdiscussed above, this arrangement offers a very small contact area, onlyat the nanowires ends between the electrolyte and the anode activematerial, thus making sacrifices in battery rate performance.

Thus, there is a tremendous need for the development of an anode/electrolyte system for these promising metal, alloy, and oxide activeanode materials so that lithium batteries can realize their fullpotential. If it were possible to use these high theoretical capacitymaterials reversibly without degradation in battery performance,substantial increases in the energy densities of lithium batteries wouldbe possible.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and others will be readily appreciated by theskilled artisan from the following description of illustrativeembodiments when read in conjunction with the accompanying drawings.

FIGS. 1 a-1 c are schematic cross-section illustrations of a particle ofanode active material as it undergoes lithium insertion.

FIG. 2 is a schematic cross-section illustration of an active materialnanoparticle embedded in an electrode as it undergoes lithiation.

FIG. 3 shows schematic cross section illustrations of alloynanoparticles embedded in a solid polymer electrolyte before (A) andafter (B) insertion of lithium.

FIG. 4 shows schematic cross section illustrations of novel electrodeswhose active material is in the form of (A) nanoparticles and (B) linearnanostructures.

FIG. 5 is a schematic cross-section illustration of a battery cell witha novel negative electrode, according to an embodiment of the invention.

FIG. 6 is a schematic drawing of a diblock copolymer and a domainstructure it can form.

FIG. 7 is a schematic drawing of a triblock copolymer and a domainstructure it can form.

FIG. 8 is a schematic drawing of a triblock copolymer and a domainstructure it can form.

DETAILED DESCRIPTION

The preferred embodiments are illustrated in the context of electrodesin a lithium battery. The skilled artisan will readily appreciate,however, that the materials and methods disclosed herein will haveapplication in a number of other contexts where ion insertion into anodematerials is desirable, particularly where accommodation of volumechange is important.

In accordance with an aspect of the present invention, the needdescribed above can be met with a novel anode that uses anionically-conductive solid polymer material as both binder andelectrolyte in combination with high-capacity anode active materialparticles. The solid polymer electrolyte material has materialproperties that allow it to stretch and a configuration that allows itto contract as the active material particles swell and shrink withinsertion and extraction of lithium. The solid polymer electrolyte isable to transfer the increased volume to voids within the anode and toretain adhesion with the active material particles, even as they undergosignificant volume changes during charge/discharge cycles of thebattery.

In accordance with another aspect of the invention, a lithium secondarybattery that employs the novel anode is provided.

These and other objects and advantages of the present invention willbecome more fully apparent from the following description taken inconjunction with the accompanying drawings.

In this disclosure, the terms “negative electrode” and “anode” are bothused to mean “negative electrode”. Likewise, the terms “positiveelectrode” and “cathode” are both used to mean “positive electrode”.

The term “solid polymer” is used herein to mean solid and semi solid(gel) materials that can support and maintain a porous void structure.pores are compressible.

The term “nanostructure” is used herein to mean a structure that has atleast two dimensions in the nanometer range. An exemplary nanostructurecan be approximately equiaxed, such as a nanoparticle or nanosphere; orit can be linear, such as a nanorod, nanofiber, nanotube, or nanowire.

In some arrangements, approximately equiaxed nanostructures have adiameter between about 5 nm and 1 μm. In some arrangements,approximately equiaxed nanostructures have a diameter between about 5 nmand 500 nm. In some arrangements, approximately equiaxed nanostructureshave a diameter between about 5 nm and 100 nm. In some arrangements,approximately equiaxed nanostructures have a diameter between about 5 nmand 50 nm.

In some arrangements, linear nanostructures have a diameter betweenabout 1 nm and 1 μm. In some arrangements, linear nanostructures have adiameter between about 1 nm and 500 nm. In some arrangements, linearnanostructures have a diameter between about 1 nm and 100 nm.

The embodiments of the invention are described most often using theexample of silicon as the nanostructured active anode material. Itshould be understood that the embodiments of the invention can also findutility when other metals, alloys, or metal oxides, which can formwell-defined intermetallic/intercalation phases with lithium, are usedinstead of the silicon. Examples of appropriate nanostructure materialinclude, but are not limited to, any one or more of metals such as,aluminum (Al), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb),magnesium (Mg), copper, nickel or alloys or mixtures thereof; Si alloyswith elements such as tin (Sn), nickel (Ni), copper (Cu), iron (Fe),cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag),titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium(Cr); silicon oxides and carbides; alloys such as Cu—Sn, Sb—Sn; andmetal oxides such as SnO₂.

Lithium is stored in an anode host material in ionic, not atomic, form.Thus, the lithium packing densities within the alloys also reflect thetheoretical charge capacities of the alloys. Table 1 shows the lithiumvolumetric packing density in unalloyed, metallic lithium, as well as ina few lithium alloys of interest:

TABLE 1 Lithium Packing Material Density (mol L⁺⁺¹) Metallic Li 76.36Li₂₂Si₅ 88.56 Li₂₂Sn₅ 75.47 Li₂₂Pb₅ 72.17

The Li packing densities in the alloys shown in Table 1 are close to theLi packing density in lithium metal. In one case, Li₂₂Si₅, the Lipacking density is even higher than it is in metallic lithium. Thecycling efficiency of lithium metal is only about 99%, so that evenafter just 100 cycles, the amount of lithium available to the batteryfalls to 36%. Thus it is customary to provide as much as three to fourtimes excess lithium to prolong cell life.

Some data suggest that lithium alloy electrodes may have higher lithiumalloy cycling efficiencies than do lithium metal electrodes. In thatcase, there would be less need to add excess active material to theelectrode. Thus, lithium alloy electrodes may be able to provide evenmore useful lithium in a smaller volume than indicated by the lithiumpacking densities shown in Table 1. Volumetric energy density isespecially important in consumer electronics, where battery volume canbe critical. Anode materials based on lithium alloys also have theadvantage that lithium dendrite formation during cell charging does notoccur as it does with metallic lithium anodes. Thus, these lithiatedalloys are very interesting candidates for the next generation oflithium battery anodes.

The main problem with using lithiated alloys as active anode material inconventional anodes is the large volume expansion they experience withlithium insertion (i.e., during cell charging). The volume increase canbe more than 200% and even as large as 400% in some cases. As a celldischarges, lithium is extracted from the active material, and thematerial shrinks. Thus, as the cell is cycled, the anode active materialexperiences repeated expansion and contraction. In conventional anodes,active material particles are held together with a rigid binder, such asPVDF (polyvinyl difluoride), to form an open network or matrix that canbe filled with liquid electrolyte. It is important to fill the networkwith as much liquid electrolyte as possible in order to maximize theactive material/electrolyte contact area. But, such a structure isincompressible and cannot accommodate any changes in active materialvolume. Thus, the active material particles and the binder holding themin place experience very large stresses during cycling. This has led tothree undesirable outcomes: disintegration of the anode active materialparticles, exfoliation of the polymeric binder at the particle surfaces,and breaking up of the binder that holds the anode together.

In general, metallic and intermetallic anode host materials, other thanlayer materials such as graphite, break into fragments after a fewcycles of lithium insertion and extraction due to the large volumechanges that occur during these processes. The fragments can be isolatedand may no longer be able to participate in the electrochemicalreactions of the anode. In addition, as the particles expand, stress istransmitted from the particles to the binder matrix. Such stress cancause non-reversible plastic failure in the binder, leading to loss ofoverall anode film integrity.

As an anode is charged, lithium penetrates small active anode materialparticles within the anode. FIG. 1 a shows one such active materialparticle 100 before charging. The particle 100 has an outside surface105. In an exemplary embodiment, the particle 100 is silicon. In FIG. 1b, Li⁺ ions 110 are about to penetrate through the surface 105 tolithiate the particle 100. FIG. 1 c shows the particle 100 at some timeafter litiation has begun. Li⁺ ions have penetrated the surface 105 anddiffused into the particle 100, inserting themselves into the siliconmatrix to form a series of Si—Li mixtures or phases 130. The originalunlithiated silicon particle 100 occupies a much smaller volume than anyof the continuously-forming Si—Li mixtures or phases 130. In general,the volume increase is a function of the Li⁺ concentration. At full Li⁺capacity, the volume increase can be as much as 400%.

FIG. 2A is a schematic cross-section illustration of a siliconnanoparticle 210 in a typical anode. At first, the active materialnanoparticle 210 contains little or no lithium. The nanoparticle 210 issurrounded by a non-compressible environment 220, made up of somecombination of binder, liquid electrolyte, and, optionally, conductivecarbon particles such as carbon black. For ease of illustration, nodistinction has been made among the various components of the noncompressible environment 220. Of course, an anode contains very manynanoparticles dispersed within the binder and electrolyte. The entirebattery, and, by extension, the anode, is enclosed ultimately by itspackaging, which forms a rigid boundary for the entire battery volume.For the purpose of this illustration, the local environment 220 is shownto end at a rigid boundary indicated by line 250, to represent theenvironment experienced by the nanoparticle 210.

As shown in FIG. 2B, lithium insertion into the silicon nanoparticle 210causes the nanoparticle to swell to a larger size as indicated by thedashed line 230. Gray arrows show the direction of expansion of thenanoparticle 210. The non-compressible and rigidly bound environment 220cannot accommodate the increasing size of the silicon nanoparticle 210and pushes back with an equal and opposite force, as indicated by theblack arrows.

The stress caused by this process can cause the silicon nanoparticle 210and/or binder in the environment 220 to crack. If the siliconnanoparticle 210 breaks apart, there is irreversible capacity loss;there is lithium tied up in the broken silicon, and that lithium can nolonger cycle between the positive and negative electrodes. Furthermore,there is less total active material available to receive cyclinglithium, as portions of the broken silicon are no longer activelyconnected to the ionic percolation network within the electrode. Ifthere is no free volume for accommodating the expansion of the siliconnanoparticle, the surrounding binder/active material/carbon composite220 may undergo non-reversible deformation as the stress increasesbeyond the yield stress of the composite, resulting in loss ofmechanical integrity of the anode film.

There have been a number of studies done to determine the fractureproperties of nanoparticles. When a particle is larger than its criticalfracture length, a fracture can move through the particle. The criticalfracture length is determined by material properties; it is directlyproportional to the square of the fracture toughness, and inverselyproportional to the square of the yield stress. For materials thatundergo brittle failures (ceramics, glasses etc.), fracture occursthrough nucleation of cracks and their subsequent propagation alongdislocations. Dislocations, however, are statistical in nature; reducingthe size of a particle also reduces the number of dislocations it cancontain. Dislocations are extremely rare in nanocrystals, e.g.,crystalline particles that are smaller than about 20 nm. By makingactive material particles very small, smaller than the critical fracturelength, the probability of fracture can be reduced. This suggests thatvarious useful nanostructured geometries, such as nanorods, nanowires,nanotubes, nanoparticles can be used to mitigate disintegration of anodeactive materials. The idea of using nano-sized intercalation hosts toprevent fracturing of the host materials has been demonstrated byseveral researchers. See for example, Gao, B., Sinha, S., Fleming, L. &Zhou, O., Adv. Mater. 13, 816-819 (2001) or Kasavajjula, U. et al.Journal of Power Sources 163, 1003-1039 (2007), which is included byreference herein.

Among nanostructured anode materials, linear structures have receivedconsiderable attention because they seem to provide improved stressdistribution and percolation for electronic transport. Nevertheless,even when anode active material particles do not crack, there has notyet been a good way for an anode to accommodate the volume increase ofthe particles upon lithium insertion. The overall failure of anode filmsbecause of stress transfer between the particles and the binder materialis a more difficult problem to solve. As has been discussed above, arigid binder matrix filled with either a liquid or solid electrolyte isincompressible and prone to cracking as a result of the stress.

Another problem that limits the use of metallic and intermetallic anodehost material particles is that during cycling, a stable solidelectrolyte interface (SEI) protective layer is not known to form on theparticle surfaces. Using nanoparticle-sized active material inherentlyexposes a large active surface area, which in turn means a large overallrate of electrolyte reduction at the surface in the absence of aprotective layer on the anode particles. As discussed above forgraphitic anodes, organic electrolytes undergo a surface reaction withthe graphite particles during the first few charge/discharge cycles,thus forming an SEI layer on the graphitic surfaces. Although the SEIlayer removes some amount of lithium from electrochemical cycling, italso protects against further reduction of the electrolyte and furtherloss of lithium. Even if a somewhat stable SEI layer can be formed onhigh capacity alloy particles during the first few cycles, as theparticles expand and contract, the SEI layer would crack, exposing freshsurfaces that are available for continued electrolyte reductionreactions and removal of additional lithium from cycling. Hence, highcapacity alloys exhibit continual, irreversible capacity loss, cancelingout the advantages that are expected from materials with such hightheoretical capacities.

FIG. 3A shows silicon nanoparticles 310 surrounded by a solid polymerelectrolyte 320 which provides a conductive path for ions to reach thenanoparticles 310 in an anode, according to an embodiment of theinvention. The solid electrolyte 320 also acts as a binder, holding thenanoparticles in well-dispersed positions. In one embodiment of theinvention, no additional binder material is used, thereby significantlyreducing or eliminating the f inactive material in the electrode. Inanother embodiment of the invention, additional binder material, such asPVDF (polyvinylidene fluoride) (not shown), is used to ensure that thenanoparticles 310 are fixed in a well-dispersed arrangement. The solidelectrolyte 320 also contains voids, one of which 340 is shown in FIG.3A. In some arrangements, the electrolyte 320 also contains conductivecarbon particles such as carbon black (not shown) to provide electronicconductivity. In some arrangements, no carbon black is used because theelectrode already has sufficient electronic conductivity. As discussedabove for FIG. 2, a rigid boundary 350 is shown around the solid polymerelectrolyte 320 for ease of illustration. At first, siliconnanoparticles 310 contain little or no lithium. As lithium penetratesinto the silicon nanoparticles 310, the nanoparticles swell to largersizes as indicated by the dashed lines 330. The solid electrolyte 320has sufficient elasticity that it can deform reversibly in response toexpansion of the nanoparticles 310. But, of course, the solidelectrolyte 320 is incompressible. If there were no void 340 within theboundary 350, there would be no space to which the electrolyte 320 couldmove, and the electrolyte 320 would push back on the particles 310,causing stress and cracking in the particles 310 and/or the solidelectrolyte 320, as has been discussed above in FIG. 2.

In the embodiment of the invention shown in FIGS. 3A and 3B, the void340 in the electrolyte 320 is compressible. As the nanoparticles 310incorporate Li ions and swell, the electrolyte 320 maintains contactwith the nanoparticles 310 as it transfers the volume expansion to thevoid 340. As shown in FIG. 3B, fully-lithiated active materialnanoparticles have swollen to their largest size 330, and the void 340has shrunk to a smaller size 345 without breaching the rigid boundary350. When the nanoparticles 310 release the Li ions, their volumedecreases. The solid electrolyte 320, in turn, pulls away from the void345, increasing the volume of the void 345. The solid electrolyte 320has good adhesion to the nanoparticles 310 and thus maintains goodcontact with the surfaces of the nanoparticles 310 as the nanoparticles310 contract. Thus the void 340/345 enables the anode to accommodate thechanges in the volume of the nanoparticles 310/330 as lithium isinserted into and extracted from the nanoparticles 310/330.

FIG. 4A shows a portion of a novel electrode 405, according to anembodiment of the invention. The electrode 405 contains siliconnanoparticles 410 embedded in a solid polymer electrolyte 420. Voids 425are dispersed throughout the electrolyte 420. The anode 405 may alsocontain other components that are not shown in FIG. 4A. In onearrangement, additional binder material, such as PVDF, is used to ensurethat the nanoparticles 410 are fixed in a well-dispersed arrangement. Inone arrangement, there are electronically conductive carbon particlesdispersed in the electrolyte 420. In another arrangement, there is acurrent collector adjacent the electrode 405. Upon charging of a lithiumcell with such an anode 405, the silicon nanoparticles 410 expand asthey take up lithium ions. As discussed above, the surroundingelectrolyte 420 can deform reversibly in response to the expansion ofthe nanoparticles and transfer the volume expansion to the voids 425. Insome arrangements, more than one electrolyte can be used in theelectrode 405.

FIG. 4B shows a portion of a novel electrode 407, according to anembodiment of the invention. The electrode 407 contains linearnanostructures 412, such as silicon nanowires, embedded in a polymerelectrolyte 420. Although the linear nanostructures 412 are shownaligned, it should be understood that they can take on other ordered ordisordered arrangements. Voids 425 are dispersed throughout theelectrolyte 420. The anode 405 may also contain other components thatare not shown in FIG. 4B. In one arrangement, additional bindermaterial, such as PVDF, is used to ensure that the linear nanostructures412 are fixed in a well-dispersed arrangement. In one arrangement, thereare electronically conductive carbon particles dispersed in theelectrolyte 420. In another arrangement, there is a current collectoradjacent the electrode 407. Upon charging of a lithium cell with such ananode 407, the silicon nanowires 412 expand as take up lithium ions. Asdiscussed above, the surrounding electrolyte 420 can deform reversiblyin response to the expansion of the nanowires and transfer the volumeexpansion to the voids 425. In some arrangements, more than oneelectrolyte can be used in the electrode 407.

It should be noted that although conventional electrodes are often madeas open networks of active material, binder, and conductive carbonparticles such as carbon black, they are inactive and contain noelectrolyte until the open space in the networks is filled with liquidelectrolyte. Only after the liquid electrolyte is added is it possiblefor the electrodes to receive and release lithium ions and thus functionas an electrochemical device. Such electrodes are not designed to retainvoids after the electrolyte is added. Rather, care is taken to ensurethat there are no voids after the electrolyte is added, as any voidspace would create a region where there is no electrolyte in contactwith active material, thereby reducing the capacity of the device, ahighly undesirable outcome. Also, voids or “free volume” cannot bemaintained when a liquid electrolyte is used, as liquids cannot retain aporous structure.

Anode layers made from active material particles, solid electrolyte, andoptional (carbon) electronically conductive particles can be made in afew different ways to create anode layers with voids according toembodiments of the invention. One such method is solvent casting andanother is extrusion.

In solvent casting, active material particles, solid polymerelectrolyte, and optional (carbon) electronically conductive particlesare mixed together with a solvent that dissolves the polymer to form aslurry. The slurry contains a suspension of active material particles inthe polymer solution. The slurry is deposited (e.g., poured) onto asubstrate and allowed to dry. As the mixture dries, solvent leaves, andvoids form in the polymer electrolyte. The size and distribution of thevoids depend on a few factors such as the mechanical properties of thepolymer and the dilution of the polymer. The more dilute the polymer(i.e., the higher the ratio of solvent to polymer), the higher theproportion of void space that can be created. The more tough thepolymer, the better the polymer can retain the void structure withoutcollapsing in on itself. Another factor that can influence the voidstructure in the polymer electrolyte is the size and size distributionof the active material (and optional conductive) particles. In somearrangements, foaming agents or surfactants can be added to the slurryto facilitate void formation.

In extrusion, a mixture of active material particles, polymerelectrolyte, and optional (carbon) electronically conductive particlesare fed into an extruder. The temperature of the extrusion is chosenabove the melting temperature/glass transition temperature of themixture. As the mixture is fed into the extruder, and the polymer meltsin the extruder, an inert gas, such as argon, is added to make voids inthe extruded anode. Again, the more tough the polymer, the better thepolymer can retain the void structure without collapsing in on itself.In some arrangements, foaming agents or surfactants can be added to theslurry to facilitate void formation. This method is also known as foamextrusion.

FIG. 5 is a schematic cross-section illustration of a battery cell 500with a novel negative electrode 505, according to an embodiment of theinvention. The negative electrode 505 is as has been described above inany of the embodiments of the invention. The cell 500 also has apositive electrode 560. There is an electrolyte layer 570 between thenegative electrode 505 and the positive electrode 560. In somearrangements, there can be a current collector 580 adjacent the positiveelectrode 560 and/or a current collector 590 adjacent the negativeelectrode 505.

There are a variety of materials that are suitable for use as solidpolymer electrolyte binders in the novel electrode as described above.In general, any polymer electrolyte that is solid or semi-solid at celloperating temperatures, has good ionic conductivity, has sufficientelasticity that it can deform reversibly and can adhere to anode activematerial particles can be used.

The electrochemical stability of the electrolyte in the anode inrelation to the active anode material is very important as reactions canremove both active anode material and lithium from active participationin the electrochemistry of the cell and thus result in capacity fade forthe cell. In general, solid polymer electrolytes are reductively morestable compared to liquid electrolytes.

In one embodiment of the invention, the solid polymer electrolyte usedin the anode or in any other part of the cell can be a polyethyleneoxide, a polysulfone, a polyacrylonitrile, a siloxane, a polyether, apolyamine, a linear copolymers containing ethers or amines, an ethylenecarbonate, a polysiloxane grafted with small molecules or oligomers thatinclude polyethers and/or alkylcarbonates or any other polymers orcombination of polymers that have a high enough ionic conductivity, whenmixed with an appropriate salt, to act as an electrolyte in a cell, suchas a lithium ion cell. Salts listed below as appropriate for blockcopolymer electrolyte can also be used with any of these electrolytes.Another example of a suitable solid polymer electrolyte is Nafion®.Polymers that are liquid at battery cell operating temperatures can bemade solid through cross-linking, as is well known in the art.

In another embodiment of the invention, the solid polymer electrolytecan be replaced by a ceramic-based electrolyte, such as lithiumphosphorous oxynitride or nascion.

In yet another embodiment of the invention, the solid polymerelectrolyte in the novel electrode is a block copolymer electrolyte.

Nanostructured Block Copolymer Electrolytes

As described in detail above, a block copolymer electrolyte can be usedin the embodiments of the invention.

FIG. 6A is a simplified illustration of an exemplary diblock polymermolecule 600 that has a first polymer block 610 and a second polymerblock 620 covalently bonded together. In one arrangement both the firstpolymer block 610 and the second polymer block 620 are linear polymerblocks. In another arrangement, either one or both polymer blocks 610,620 has a comb (or branched) structure. In one arrangement, neitherpolymer block is cross-linked. In another arrangement, one polymer blockis cross-linked. In yet another arrangement, both polymer blocks arecross-linked.

Multiple diblock polymer molecules 600 can arrange themselves to form afirst domain 615 of a first phase made of the first polymer blocks 610and a second domain 625 of a second phase made of the second polymerblocks 620, as shown in FIG. 6B. Diblock polymer molecules 600 canarrange themselves to form multiple repeat domains, thereby forming acontinuous nanostructured block copolymer material 640, as shown in FIG.6C. The sizes or widths of the domains can be adjusted by adjusting themolecular weights of each of the polymer blocks.

In one arrangement the first polymer domain 615 is ionically conductive,and the second polymer domain 625 provides mechanical strength to thenanostructured block copolymer.

FIG. 7A is a simplified illustration of an exemplary triblock polymermolecule 700 that has a first polymer block 710 a, a second polymerblock 720, and a third polymer block 710 b that is the same as the firstpolymer block 710 a, all covalently bonded together. In one arrangementthe first polymer block 710 a, the second polymer block 720, and thethird copolymer block 710 b are linear polymer blocks. In anotherarrangement, either some or all polymer blocks 710 a, 720, 710 b have acomb (or branched) structure. In one arrangement, no polymer block iscross-linked. In another arrangement, one polymer block is cross-linked.In yet another arrangement, two polymer blocks are cross-linked. In yetanother arrangement, all polymer blocks are cross-linked.

Multiple triblock polymer molecules 700 can arrange themselves to form afirst domain 715 of a first phase made of the first polymer blocks 710a, a second domain 725 of a second phase made of the second polymerblocks 720, and a third domain 715 b of a first phase made of the thirdpolymer blocks 710 b as shown in FIG. 7B. Triblock polymer molecules 700can arrange themselves to form multiple repeat domains 425, 415(containing both 415 a and 415 b), thereby forming a continuousnanostructured block copolymer 730, as shown in FIG. 7C. The sizes ofthe domains can be adjusted by adjusting the molecular weights of eachof the polymer blocks.

In one arrangement the first and third polymer domains 715 a, 715 b areionically conductive, and the second polymer domain 725 providesmechanical strength to the nanostructured block copolymer. In anotherarrangement, the second polymer domain 725 is ionically conductive, andthe first and third polymer domains 715 provide a structural framework.

FIG. 8A is a simplified illustration of another exemplary triblockpolymer molecule 800 that has a first polymer block 810, a secondpolymer block 820, and a third polymer block 830, different from eitherof the other two polymer blocks, all covalently bonded together. In onearrangement the first polymer block 810, the second polymer block 820,and the third copolymer block 830 are linear polymer blocks. In anotherarrangement, either some or all polymer blocks 810, 820, 830 have a comb(or branched) structure. In one arrangement, no polymer block iscross-linked. In another arrangement, one polymer block is cross-linked.In yet another arrangement, two polymer blocks are cross-linked. In yetanother arrangement, all polymer blocks are cross-linked.

Multiple triblock polymer molecules 800 can arrange themselves to form afirst domain 815 of a first phase made of the first polymer blocks 810a, a second domain 825 of a second phase made of the second polymerblocks 820, and a third domain 835 of a third phase made of the thirdpolymer blocks 830 as shown in FIG. 8B. Triblock polymer molecules 800can arrange themselves to form multiple repeat domains, thereby forminga continuous nanostructured block copolymer 840, as shown in FIG. 8C.The sizes of the domains can be adjusted by adjusting the molecularweights of each of the polymer blocks.

In one arrangement the first polymer domains 815 are ionicallyconductive, and the second polymer domains 825 provide mechanicalstrength to the nanostructured block copolymer. The third polymerdomains 835 provides an additional functionality that may improvemechanical strength, ionic conductivity, chemical or electrochemicalstability, may make the material easier to process, or may provide someother desirable property to the block copolymer. In other arrangements,the individual domains can exchange roles.

Choosing appropriate polymers for the block copolymers described aboveis important in order to achieve desired electrolyte properties. In oneembodiment, the conductive polymer (1) exhibits ionic conductivity of atleast 10⁻⁵ Scm⁻¹ at electrochemical cell operating temperatures whencombined with an appropriate salt(s), such as lithium salt(s); (2) ischemically stable against such salt(s); and (3) is thermally stable atelectrochemical cell operating temperatures. In one embodiment, thestructural material has a modulus in excess of 1×10⁵ Pa atelectrochemical cell operating temperatures. In one embodiment, thethird polymer (1) is rubbery; and (2) has a glass transition temperaturelower than operating and processing temperatures. It is useful if allmaterials are mutually immiscible.

In one embodiment of the invention, the conductive phase can be made ofa linear or branched polymer. Conductive linear or branched polymersthat can be used in the conductive phase include, but are not limitedto, polyethers, polyamines, polyimides, polyamides, alkyl carbonates,polynitriles, and combinations thereof. The conductive linear orbranched polymers can also be used in combination with polysiloxanes,polyphosphazines, polyolefins, and/or polydienes to form the conductivephase.

In another exemplary embodiment, the conductive phase is made of comb(or branched) polymers that have a backbone and pendant groups.Backbones that can be used in these polymers include, but are notlimited to, polysiloxanes, polyphosphazines, polyethers, polydienes,polyolefins, polyacrylates, polymethacrylates, and combinations thereof.Pendants that can be used include, but are not limited to, oligoethers,substituted oligoethers, nitrile groups, sulfones, thiols, polyethers,polyamines, polyimides, polyamides, alkyl carbonates, polynitriles,other polar groups, and combinations thereof.

Further details about polymers that can be used in the conductive phasecan be found in U.S. Provisional Patent Application No. 61/056,688,filed May 28, 2008, U.S. Provisional Patent Application No. 61/091,626,filed Aug. 25, 2008, U.S. Provisional Patent Application No. 61/145,518filed Jan. 16, 2009, U.S. Provisional Patent Application No. 61/145,507,filed Jan. 16, 2009, U.S. Provisional Patent Application No. 61/158,257filed Mar. 6, 2009, and U.S. Provisional Patent Application No.61/158,241, filed Mar. 6, 2009, all of which are included by referenceherein.

Further details about polymers that can be used in the conductive phasecan be found in International Application Number PCT/US09/45356, filedMay 27, 2009, U.S. Provisional Patent Application No. 61/091,626, filedAug. 25, 2008, U.S. Provisional Patent Application No. 61/145,518, filedJan. 16, 2009, U.S. Provisional Patent Application No. 61/145,507, filedJan. 16, 2009, U.S. Provisional Patent Application No. 61/158,257, filedMar. 6, 2009, and U.S. Provisional Patent Application No. 61/158,241,filed Mar. 6, 2009, all of which are included by reference herein.

There are no particular restrictions on the electrolyte salt that can beused in the block copolymer electrolytes. Any electrolyte salt thatincludes the ion identified as the most desirable charge carrier for theapplication can be used. It is especially useful to use electrolytesalts that have a large dissociation constant within the polymerelectrolyte.

Suitable examples include alkali metal salts, such as Li salts. Examplesof useful Li salts include, but are not limited to, LiPF₆, LiN(CF₃SO₂)₂,Li(CF₃SO₂)₃C, LiN(SO₂CF₂CF₃)₂, LiB(C₂O₄)₂, B₁₂F_(x)H_(12-x), B₁₂F₁₂, andmixtures thereof.

In one embodiment of the invention, single ion conductors can be usedwith electrolyte salts or instead of electrolyte salts. Examples ofsingle ion conductors include, but are not limited to sulfonamide salts,boron based salts, and sulfates groups.

In one embodiment of the invention, the structural phase can be made ofpolymers such as polystyrene, polymethacrylate, poly(methylmethacrylate), polyvinylpyridine, polyvinylcyclohexane, polyimide,polyamide, polypropylene, polyolefins, poly(t-butyl vinyl ether),poly(cyclohexyl methacrylate), poly(cyclohexyl vinyl ether),poly(t-butyl vinyl ether), polyethylene, fluorocarbons, such aspolyvinylidene fluoride, or copolymers that contain styrene,methacrylate, or vinylpyridine.

Additional species can be added to nanostructured block copolymerelectrolytes to enhance the ionic conductivity, to enhance themechanical properties, or to enhance any other properties that may bedesirable.

The ionic conductivity of nanostructured block copolymer electrolytematerials can be improved by including one or more additives in theionically conductive phase. An additive can improve ionic conductivityby lowering the degree of crystallinity, lowering the meltingtemperature, lowering the glass transition temperature, increasing chainmobility, or any combination of these. A high dielectric additive canaid dissociation of the salt, increasing the number of Li+ ionsavailable for ion transport, and reducing the bulky Li+[salt] complexes.Additives that weaken the interaction between Li+ and PEO chains/anions,thereby making it easier for Li+ ions to diffuse, may be included in theconductive phase. The additives that enhance ionic conductivity can bebroadly classified in the following categories: low molecular weightconductive polymers, ceramic particles, room temp ionic liquids (RTILs),high dielectric organic plasticizers, and Lewis acids.

Other additives can be used in the polymer electrolytes describedherein. For example, additives that help with overcharge protection,provide stable SEI (solid electrolyte interface) layers, and/or improveelectrochemical stability can be used. Such additives are well known topeople with ordinary skill in the art. Additives that make the polymerseasier to process, such as plasticizers, can also be used.

Further details about block copolymer electrolytes are described in U.S.patent application Ser. No. 12/225,934, filed Oct. 1, 2008, U.S. patentapplication Ser. No. 12/271,1828, filed Nov. 14, 2008, and PCT PatentApplication Number PCT/US09/31356, filed Jan. 16, 2009, all of which areincluded by reference herein.

This invention has been described herein in considerable detail toprovide those skilled in the art with information relevant to apply thenovel principles and to construct and use such specialized components asare required. However, it is to be understood that the invention can becarried out by different equipment, materials and devices, and thatvarious modifications, both as to the equipment and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

We claim:
 1. An electrode for an electrochemical cell comprising: asolid polymer electrolyte comprising a plurality of voids; andnanostructures comprising electrode active material, the nanostructuresdistributed within and throughout the solid polymer electrolyte, whereinthe plurality of voids is compressible such that upon incorporation ofthe electrode active material into the nanostructures, the solid polymerelectrolyte expands into the plurality of voids.
 2. The electrode ofclaim 1, further comprising a binder to bind the nanostructures withinthe electrode.
 3. The electrode of claim 2 wherein the binder comprisesPVDF.
 4. The electrode of claim 1 wherein the electrode contains noadditional electrolytes.
 5. The electrode of claim 1 wherein the solidpolymer electrolyte has a yield strain of 50-500% and an elastic modulusof greater than 1×10⁵ Pa.
 6. The electrode of claim 1 wherein thenanostructures are selected from the group consisting of nanorods,nanowires, nanotubes, and nanoparticles.
 7. The electrode of claim 1wherein the nanostructures comprise carbon, aluminum, silicon,germanium, tin, lead, antimony, magnesium, copper, nickel or alloys ormixtures thereof.
 8. The electrode of claim 1 wherein the nanostructurescomprise silicon.
 9. The electrode of claim 1 wherein the nanostructurescomprise silicon alloys of tin (Sn), nickel (Ni), copper (Cu), iron(Fe), cobalt (Co), manganese (Mn), zinc (Zn), indium (In), silver (Ag),titanium (Ti), germanium (Ge), bismuth (Bi), antimony (Sb), and chromium(Cr) or mixtures thereof.
 10. The electrode of claim 1 wherein thenanostructures comprise silicon oxides or silicon carbides.
 11. Theelectrode of claim 1 wherein the nanostructures comprise a metal oxide.12. The electrode of claim 1 wherein the nanostructures have nodimension larger than their critical fracture length.
 13. The electrodeof claim 1 wherein the nanostructures have no more than one dimensionlarger than their critical fracture length.
 14. The electrode of claim 1wherein the nanostructures are approximately equiaxed and have adiameter between about 5 nm and 1 μm.
 15. The electrode of claim 1wherein the nanostructures are approximately equiaxed and have adiameter between about 5 nm and 500 nm.
 16. The electrode of claim 1wherein the nanostructures are approximately equiaxed and have adiameter between about 5 nm and 100 nm.
 17. The electrode of claim 1wherein the nanostructures are approximately equiaxed and have adiameter between about 5 nm and 50 nm.
 18. The electrode of claim 1wherein the nanostructures are linear and have a diameter between about1 nm and 1 μm.
 19. The electrode of claim 1 wherein the nanostructuresare linear and have a diameter between about 1 nm and 500 nm.
 20. Theelectrode of claim 1 wherein the nanostructures are linear and have adiameter between about 1 nm and 100 nm.
 21. The electrode of claim 1wherein the plurality of voids has a total volume no smaller than fourtimes the total volume of the nanostructures before lithiation.
 22. Theelectrode of claim 1 wherein the plurality of voids has a total volumeno smaller than three times the total volume of the nanostructuresbefore lithiation.
 23. The electrode of claim 1 wherein the plurality ofvoids has a total volume no smaller than two and a half times the totalvolume of the nanostructures before lithiation.
 24. The electrode ofclaim 1 wherein the plurality of voids has a total volume no smallerthan twice the total volume of the nanostructures before lithiation. 25.The electrode of claim 1 wherein the voids have a volume that is betweenabout 10% and 60% of the electrode.
 26. The electrode of claim 1,further comprising between about 0 and 10 weight percent conductivecarbon particles.
 27. The electrode of claim 1 wherein the solid polymerelectrolyte comprises one or more polymers selected from the groupconsisting of the following optionally cross-linked polymers:polyethylene oxide, polysulfone, polyacrylonitrile, siloxane, polyether,polyamine, linear copolymers containing ethers or amines, ethylenecarbonate, Nafion®, and polysiloxane grafted with small molecules oroligomers that include polyethers and/or alkylcarbonates.
 28. Theelectrode of claim 1 wherein the solid polymer electrolyte comprises ablock copolymer.
 29. The electrode of claim 28 wherein the solid polymerelectrolyte further comprises at least one lithium salt.
 30. Theelectrode of claim 28 wherein the block copolymer is either a diblockcopolymer or a triblock copolymer.
 31. The electrode of claim 30 whereina first block of the block copolymer is ionically conductive and isselected from the group consisting of polyethers, polyamines,polyimides, polyamides, alkyl carbonates, polynitriles, polysiloxanes,polyphosphazines, polyolefins, polydienes, and combinations thereof. 32.The electrode of claim 30 wherein a first block of the block copolymercomprises an ionically-conductive comb polymer, which comb polymercomprises a backbone and pendant groups.
 33. The electrode of claim 32wherein the backbone comprises one or more selected from the groupconsisting of polysiloxanes, polyphosphazines, polyethers, polydienes,polyolefins, polyacrylates, polymethacrylates, and combinations thereof.34. The electrode of claim 32 wherein the pendants comprise one or moreselected from the group consisting of oligoethers, substitutedoligoethers, nitrile groups, sulfones, thiols, polyethers, polyamines,polyimides, polyamides, alkyl carbonates, polynitriles, other polargroups, and combinations thereof.
 35. The electrode of claim 30 whereina second block of the block copolymer is selected from the groupconsisting of polystyrene, polymethacrylate, poly(methyl methacrylate),polyvinylpyridine, polyvinylcyclohexane, polyimide, polyamide,polypropylene, polyolefins, poly(t-butyl vinyl ether), poly(cyclohexylmethacrylate), poly(cyclohexyl vinyl ether), poly(t-butyl vinyl ether),polyethylene, fluorocarbons, polyvinylidene fluoride, and copolymersthat contain styrene, methacrylate, and/or vinylpyridine.
 36. A lithiumbattery cell, comprising: a negative electrode according to theelectrode of claim 1; a positive electrode; a solid polymer electrolytelayer between the negative electrode and the positive electrode, theelectrolyte layer in ionic communication with both the negativeelectrode and the positive electrode.
 37. The electrode of claim 1wherein the plurality of voids does not contain the nanostructures. 38.The electrode of claim 28 wherein the block copolymer is nanostructured.39. The electrode of claim 28 wherein one or more blocks of the blockcopolymer are cross-linked.